The proposed research brings cutting-edge genetic tools to bear on understanding the neural circuitry under- lying aggression and motivated behavior. Our recent progress reveals that these behaviors in mice are regulated by two molecularly distinct, small subsets of brain serotonergic (5-HT) neurons: one subset, ~3,000 neurons, uniquely defined among 5-HT neurons by expression of the D1 dopamine (DA) receptor (Drd1a gene), and the other, ~1,000 neurons, by the D2 DA receptor (Drd2 gene). These results combined with the enabling genetic tools, like a powerfully sharpened wedge, can now be used to break open and access the circuitry, cellular properties, and molecular pathways underlying these consequential behaviors. Here we propose applying this wedge in the form of four aims to answer: What cellular and molecular properties are unique to these behavior-critical 5-HT neurons? As suggested by receptor expression, are these 5-HT neurons responsive to DA - a neurochemical commonly associated with the reward system of the brain? Through what forebrain circuitry do these subtypes modulate aggression and motivation? Do these parameters change across the life span, perhaps bearing on human age-related propensity for impulsivity, aggression, and substance abuse? In Aim 1, we will identify functional properties of the Drd1a and Drd2 5-HT neuron subtypes by transcriptional profiling (RNA-seq) and electrophysiological recording. This work is enabled through novel genetic tools for neuron subtype-specific marking, suitable for neuron subtype sorting and molecular profiling, and for whole-cell recording. These same genetic tools not only offer access to the soma of a 5-HT neuron subtype, but also to axons and terminals, thus allowing precise identification of target brain regions under neuron-subtype control - the goal of Aim 2. Functional postsynaptic connections will also be explored, with our intersectional genetic marking tools conferring unprecedented resolution to classic tract- tracing techniques as well as to cutting-edge viral approaches that involve trans-synaptic tracers. Thus, Aim 2 will define, label, and allow for molecular characterization of aggression-relevant postsynaptic neurons downstream in these circuits. In Aim 3, we will explore more deeply the behavioral facets modulated by these two 5-HT neuron subtypes and if their contributions vary across life span. Similar subtype-specific silencing methods will be employed as in the foundational aggression studies, but now additional social behaviors and neurological functions will be queried. In Aim 4, we will use pharmacogenetics (DREADDs) to transiently silence each Drd 5-HT neuron subtype during childhood, asking if lasting changes occur that predispose to hyperaggression and altered social motivation in adulthood, as predicted by human studies that associate genetic predisposition via the 5-HT system, childhood stress, and adult pathological aggression. Our approaches are technically and conceptually innovative, and are foundational for discovering new, potentially behavior-selective, age-suitable therapeutics. Results compel a redefinition of 5-HT system organization.